This invention relates generally to electroluminescent (EL) lamp drivers and, more particularly, to a circuit and method for indirectly regulating the output voltage of such EL lamp drivers.
The prior art direct method of controlling the output voltage of an EL lamp driver involves first measuring the output voltage and comparing it to a reference voltage. Then, the error signal generated by the difference between those two voltages is employed in a negative feedback control circuit to regulate the output voltage. A typical prior art EL lamp driver circuit and the external components required to properly power an EL panel, are illustrated in FIG. 1. This prior art circuit converts a low battery voltage (1.5-12.0 volts) supplied by a battery 101 to a high AC voltage (60-180 volts) that is used to drive a fixed capacitive load 105 of an EL lamp panel 105. This high AC voltage level applied across the EL lamp panel is the primary factor in determining the brightness of the EL panel. The changes in light intensity due to small output voltage changes are typically minimally perceptible to the eye.
Exemplary of the commercially available prior art EL lamp drivers 100 of FIG. 1 are the HV803 from Suptertex, Inc., the IMP803 from IMP, Inc., and the similar circuits described in U.S. Pat. Nos. 5,463,283 and 5,717,317. A typical block diagram of these prior art lamp drivers of FIG. 1 is shown in FIG. 2. The circuit of FIG. 2 comprises a boost converter section 209 and an H-bridge driver section 210. A detailed circuit diagram of the boost converter section 209 is illustrated in FIG. 3. In operation, a switch oscillator 200 drives a FET switch 201 to its on state for a fixed duty cycle of 88%. During the on time of the switch 201, energy is being stored in an inductor 102, and during the off time, this stored energy is substantially transferred to an output capacitor 103 through a catch diode 104. As energy builds in capacitor 103, the voltage V(OUT) across the capacitor 103 rises. Regulation of V(OUT) occurs when it rises above the regulation voltage such that the voltage presented at the mid-point of the voltage divider resistors 205, 206 is equal to the voltage V(REF) 207. At this point the comparator 208 changes state and disables the switch oscillator 200. The energy in the output capacitor 103 is then discharged by the H-bridge load 309, until the voltage V(OUT) decays below the regulation voltage, causing the switch oscillator 200 to again recharge the output capacitor 103. The regulation method employed in the prior art HV803 and IMP803 EL lamp drivers is known as pulse frequency modulation (PFM). PFM switching regulators are disadvantageous in that they can cause radio frequency interference (RFI) problems in wireless communication systems due to the fact that their switching operation tends to be somewhat random, depending on the input voltage of the battery power source.
The resistive divider comprising resistors 205, 206 is employed to sense the output voltage of the prior art boost converter of FIG. 3. The large amount of silicon required to fabricate this resistive divider in an integrated circuit adds to its expense and size. In addition, due to the high conversion ratio between the input and output voltages of the boost converter, the current drain through resistors 205, 206 is greatly multiplied at the input power source, thereby reducing the overall efficiency of the boost converter.
In an EL lamp driver application in which the frequency of lamp operation and the output voltage are set, the load current flowing into the H-bridge load is very predictable and relatively constant. Precise regulation of the output voltage is not necessary, because the small change in light intensity of the EL panel resulting from a small change in output voltage caused by component inaccuracies is imperceptible to the user.
It would therefore be advantageous to provide an improved EL lamp driver that takes advantage of the fact that relatively small changes in output voltage do not result in noticeable changes in light intensity of the EL panel being driven. It would also be advantageous to provide an improved EL lamp driver that employs pulse width modulation (PWM) rather than pulse frequency modulation (PFM) as a regulation technique to thereby minimize the resulting RFI. Also, elimination of the high voltage resistive divider network of the prior art greatly reduces the silicon area required when fabricating the EL lamp driver as an integrated circuit. Such an improved EL lamp driver could thereby be fabricated as a more compact integrated circuit that would operate more efficiently than the integrated circuit EL lamp drivers known in the prior art.